Bioenergy conversion (the conversion of biomass to useful energy) is gaining in popularity as the visibility of renewable energy increases. But growing crops specifically for bioenergy conversion raises separate concerns, such as competing with land and other resources used for growing crops for food and thereby increasing the cost of food. Therefore, using organic waste material instead, such as manure, crop residues, and organic municipal wastes, has become a more attractive alternative for use in bioenergy conversion.
Aerobic and anaerobic digestion of organic material are known processes for treating waste materials. Aerobic digestion, or “composting” as it is more commonly referred to, is the processing of waste materials in the presence of air. Aerobic digesters produce compost (a soil amendment) and heat. Anaerobic digesters process waste materials in the absence of oxygen, and produce a digestate suitable for use as a soil amendment or fertilizer. Anaerobic digesters are particularly attractive for treating waste materials, because the process reduces overall volume and mass of the waste while producing biogas used for heat and/or generating electricity.
In anaerobic digestion, microbes utilize the waste material as a food source, releasing nutrients (e.g., nitrogen and phosphorus) which are suitable for use as fertilizer. The microbes also produce a mixture of gases as a byproduct of the digestion process. This biogas is high in methane content, which has a high potential for generating energy.
Several anaerobic digestion technologies exist, including complete mix, plug flow, batch, fixed film, and anaerobic sludge blanket technologies. These technologies can treat waste materials having a total solids (TS) content of less than about 15%. Yet confined animal feeding operations (e.g., dairies and feedlots) produce large quantities of animal waste having total solids content much greater than 15%. Accordingly, this waste has to be diluted with large quantities of water before the waste can be treated using convention anaerobic digestion processes.
For purposes of illustration, 1000 metric tons of waste with 90% total solids needs to be diluted with 9000 cubic meters of water in order to be treated using a conventional complete mix digestion process. Using this much water in a waste treatment process is not desirable, particularly in arid western states where water comes at a premium. Even if this much water is available for use in waste treatment processes, dilution causes heavier inorganic particles (e.g., sand and soils) to settle in the digester. “Sandbars” may form at the bottom of the digester, which interrupt stable operation and necessitate frequent cleaning. In addition, having to pump such staggering quantities of water, and constructing a digester large enough to hold all of this water, renders these treatment processes difficult to implement in a cost effective manner.
Dry digestion technologies also exist. These technologies can operate with up to 35% total solids content, and thus reduce the amount of water needed to dilute the waste materials. For purposes of illustration, 1000 metric tons of waste with 90% total solids content only needs to be diluted with 2700 cubic meters of water. Compare this volume of water to the 9000 cubic meters of water needed for the complete mix digester example discussed above. But dry digestion processes are typically implemented as batch systems. The waste is loaded into a large reactor, moistened, and covered to maintain anaerobic conditions. This results in low hydrolysis rates and thus long retention times to be effective, which drives up the operating costs.
Leachate production with recirculation is a form of dry digestion. Leachate production processes are implemented by pumping liquid onto the surface of a bed of waste material so that the liquid percolates through the bed. As the leachate drains away from the solid, more leachate is added and/or recirculated onto the surface of the bed, so that there is a constantly exchanging film of leachate surrounding the waste material during the digestion process. However, the microbes (methanogens) used in anaerobic digestion processes are sensitive to low pH levels in the leachate bed. In addition, like other batch systems, the inoculum is completely exchanged at the end of each batch and the next batch takes time to re-grow an acclimated population of microorganisms before effective digestion can occur. Thus, leachate production processes generally exhibit low methane production efficiency.